The present invention relates generally to the use of low frequency vibrations to investigate passive objects. More particularly, the present invention relates to a method of and a system for using low frequency external stimulation to vibrate an object and then using a second high frequency wave energy reflected by an object for analyzing the vibration of the object created by the generated low frequency vibrations in order to examine the surface or cross-section of the object and to produce an image of the examined object.
It is frequently useful to examine passive structures or objects by means of the application of a swept frequency vibration or audio source. Structures which are typically examined in that manner for flaws include aircraft, ships, bridge trusses and other types of large structures. In addition, soft tissue can also be examined in that manner.
In the examination of such structures, the vibration sources may be temporarily mounted on exterior surfaces of, for example, ship hulls or bridge trusses, or may be placed in the interior of the structures to be examined, such as ship hulls or passenger aircraft. The interrogating wave energy is then focused on a reference point on or within the object.
Such interrogating wave energy may vary from a laser, microwave or airborne ultrasound source, for use with aircraft, bridges, ships or other structures. Sonar or ultrasound sources may be utilized for underwater inspection. The vibration source of low frequency is swept over a broad frequency range so as to excite eigenmodes with relatively high vibrational amplitudes. Once an appropriate vibration frequency is found, the interrogating wave energy can then be scanned over the surface or within a cross-section of the interior for penetrating waves. The spot size or spatial resolution of the scan, depends upon the wavelength of the vibration source used and the particular apparatus used. For simply focused coherent sources, the spot size will be equal to (1.2)(wavelength) (focal length)/(aperture radius).
Since sub-millimeter wavelengths can be achieved by the instant invention using ultrasound and optical devices as interrogating wave sources, millimeter scale, spatial resolution can be achieved utilizing the present invention. Once an appropriate scan size or region of interest is selected, an image is derived from point-by-point examination of the Doppler shift of the reflection of the interrogating source back from the object. Although different prior techniques have been described which analyze vibrations using lasers or ultrasound, none of those techniques make use of externally applied vibration and point-by-point scanning using the method of the present invention, to generate a vibration image.
The method of the present invention is useful to generate an image whose intensity or color is proportional to the vibration amplitude calculated by means of the inventive method, at each point on the object. That image can be inspected for modal shapes and abnormally high or low vibration amplitudes, and can also be compared with reference images obtained earlier or from well characterized analogous structures. The time required for analysis of each spot utilizing the inventive method and system described herein is less than three cycles of the vibration frequency when the frequency domain estimator method is utilized and a fraction of a single vibration cycle if the time domain estimator method is utilized. Thus, those methods can be applied rapidly so as to permit real-time imaging. Because the methods are also sensitive to vibration but are stable in the presence of noise, vibrational amplitudes of less than 1/10 of a wavelength of the interrogating wave energy can be easily detected utilizing the inventive method and system.
One prior art approach to measuring amplitudes of vibration is shown in U.S. Pat. No. 4,819,649, issued Apr. 11, 1989, to Rogers et al. That patent is directed to a non-invasive vibration measurement system and method for measuring the acoustically induced vibrations within a living organism. That patent utilizes a continuous wave of high spectral purity ultrasonic beams and utilizes two separate transducers, one for transmitting and one for receiving the focused beams.
By virtue of its frequency domain processing, the device disclosed by Rogers et al. cannot produce real-time imaging. In fact, the '649 patent does not discuss imaging at all. All of the specific implementations discussed in the '649 patent relate to frequency domain techniques which are based upon the ratio of harmonic sidebands of the reflected signal, which allows the intrusion of noise elements into the reflected sample and, thus, into any analyzed signal.
The system and method disclosed in the '649 patent is also discussed in an article written by the inventors which appeared in the Journal of Vibration, Acoustics, Stress and Reliability in Design, entitled "Automated Non-Invasive Motion Measurement of Auditory Organs in Fish Using Ultrasound", Vol. 109, January 1987, pp. 55-59. That paper discloses the use of external vibration to produce an FM Doppler shift and examines, over long periods of time, the Doppler spectrum returning from a single point. It is not a real-time system. The article assumes that, using very small vibrations, the ratio of the carrier signal to the first harmonic is indicative of the vibration amplitude. Like the '649 patent, the device of Rogers and Cox disclosed in this paper is a non-scanning, slow, frequency domain method which uses the ratio of harmonic sidebands and is restricted to use with very low amplitudes.
Another approach used in the past is disclosed in an article entitled "Imaging the Amplitude of Vibration Inside the Soft Tissues for Forced Low Frequency Vibration", by Yamakoshi, Mori and Sato, published in the Japanese Journal of Medical Ultrasonics, Vol. 16, No. 3, pp. 221-229 (1989). That article discusses an imaging system which can observe the precise movements inside of soft tissues when an external vibration is applied to those tissues. While the system described in that article does scan and make vibration images, it uses frequency domain techniques which are also based on the ratio of harmonic sidebands approach which are noise sensitive. Furthermore, it is slow and not a practical real-time system and is restricted to use with small amplitudes of vibration.
Yet another approach used in the prior art is that of Pierce and Berthelot, as disclosed in Proceedings of the SPIE, Session EE. Engineering Acoustics IV: "Laboratory and Measurement Microphone", "Absolute Calibration of Acoustic Sensors Utilizing Electromagnetic Scattering from In Situ Particulate Matter", Pierce and Yves (1988). That paper describes the same FM Doppler spectrum utilized by Cox and Rogers and as described in their article discussed above. However, Pierce et al. utilize laser methods to measure the oscillation at a point. The methodology of Pierce et al. is a non-scanning, slow, frequency domain technique which again uses the ratio of harmonic sidebands and therefore suffers from the same noise sensitivity problems as do the other prior systems discussed above.
All of the known techniques can be broadly classified as utilizing the same approach to the estimation or determination of the vibrational parameters, that is, using some ratio of spectral harmonic amplitudes. Thus, they all suffer from the disadvantages of the ratio methods because they require either intensive computation or larger lookup tables of theoretical Bessel functions for comparison with the measured data. Further, ratio methods work well only when the argument of the Bessel function is small, which poses a severe limitation on the range of the estimation of the Doppler spectrum.
As a practical matter, the performance of the ratio methods is highly degraded since almost all Doppler spectra suffer from a poor signal-to-noise ratio. Additionally, a sophisticated algorithm is required to determine the best selection of the harmonic pair to be compared. The present invention, on the other hand, utilizes a simple and noise-immune method for vibration estimation or determination.